1. Field of the Invention
The present invention relates to an image information reading apparatus, and more specifically, relates to an image information reading apparatus provided with an optical head which is capable of moving at least in a primary scanning direction.
2. Description of the Related Art
A fluorescence detection system adopting fluorescent dye as a marker has been well-known in the fields of biochemistry and molecular biology. With this system, fluoroscopic image information of a sample, e.g. a gel-like sample supporting an organic substance which is distributed therein and marked with the fluorescent dye, can be read in a photoelectric manner. That enables, for example, evaluation of gene sequences, a level of gene expression, paths and stages of a metabolic process, an absorption process or an excretion process of a substance administered to a rat for an experiment, and molecular weight and other properties of a protein, as well as separation and identification of a protein.
Further, molecular weight and other properties of a DNA fragment may also be evaluated by preparing a gel-like supporting object in which a plurality of the DNA fragments marked with the fluorescent dye are distributed based on electrophoresis, a phenomenon in which living cells in a suspension or molecules of a biological compound (e.g. a protein) in a solution are collected around an anode or a cathode because of interaction between electrical charges carried by the cells or the molecules and an electric field in the suspension or the solution, irradiating the gel-like supporting object with stimulating light which is capable of stimulating the fluorescent dye used as the marker therein, photoelectrically detecting the fluorescence induced in the gel-like supporting object to obtain image information representing the distribution of the DNA fragments marked with the fluorescent dye, and displaying a visual image (an image of the above distribution which may be referred to as a fluoroscopic image) on a display device such as a display part of CRT based on the obtained image information, wherein the gel-like supporting object may be prepared by adding the fluorescent dye to a solution containing the DNA fragments and then inducing electrophoresis motion of the DNA fragments within the gel-like supporting object, by inducing the electrophoresis motion of the DNA fragments within the gel-like supporting object containing the fluorescent dye, or by inducing the electrophoresis motion of the DNA fragments within the gel-like supporting object and then soaking the gel-like supporting object under a solution containing the fluorescent dye.
An image information reading apparatus for carrying out the processes as described above has become quite popular in the fields of biochemistry and molecular biology. This image information reading apparatus irradiates the gel-like supporting object with the stimulating light, detects the fluorescence in the photoelectric manner to obtain the image information, and displays the fluoroscopic image on the display device based on the obtained image information.
Though the sample is scanned with the stimulating light using a rotating polygon mirror and an fxcex8 lens in the image information reading apparatus of the most popular type, the image information reading apparatus suggested in the present application is of another type called an optical head scanning type which does not employ the rotating polygon mirror and the fxcex8 lens. FIGS. 4 and 5 illustrate a typical constitution of the image information reading apparatus of the optical head scanning type which appears in, for example, Japanese Patent Publication No. 10(1998)-3134. The illustrated apparatus is constituted of a light source 30 for generating an stimulating light beam L, a lens 31, a photomultiplier 40 for detecting intensity of fluorescence K entering thereon in the photoelectric manner to generate an image signal S, an stimulating light cutting filter 41, a sample holding portion 20 for holding a scanning target carrying certain image information, a gel-like supporting object 10 with glass in this example, an optical head 50 which leads the stimulating light beam L emitted by the light source 30 to the gel-like supporting object 10 placed on the sample holding portion 20 so that the gel-like supporting object 10 is irradiated with the stimulating light beam L and the fluorescence K is induced thereby and which leads the fluorescence K emitted downward from the upper surface of the gel-like supporting object 10 to the photomultiplier 40, primary scanning means 60 for moving the optical head 50 in a primary scanning direction (a direction of x as indicated with an arrow in FIG. 4) with respect to the gel-like supporting object 10 so that the gel-like supporting object 10 is scanned in the primary scanning direction with the stimulating light beam L led thereto by the optical head 50, and secondary scanning means 80 for moving the optical head 50 etc. in a secondary scanning direction (a direction of Y as indicated with another arrow in FIG. 4) which is orthogonal to the primary scanning direction.
According to the image information reading apparatus of the optical head scanning type, the stimulating light beam L passes through an aperture 52a opened on the mirror 52 which constitutes the optical head 50, and then stimulates the gel-like supporting object 10 to induce the fluorescence K therein. The fluorescence K emitted downward from the upper surface of the gel-like supporting object 10 proceeds along an optical path of the stimulating light beam L in the reverse direction and reaches the mirror 52 on which the aperture 52a is opened. As a beam diameter of the fluorescence K at the mirror 52 is larger than that of the stimulating light beam L, only a small fraction of the fluorescence K passes through the aperture 52a, and a major portion of the fluorescence K is reflected by the mirror 52, led to the photomultiplier 40, and detected by the photomultiplier 40 in the photoelectric manner. The image information reading apparatus of the optical head scanning type is effective in improving an S/N ratio of the signal (the image information) read in the photoelectric manner, as the apparatus does not employ a dichroic mirror for irradiating the scanning target, e.g. the gel-like supporting object 10, and thus provides more excitation energy to the scanning target. than an apparatus in which the scanning target is irradiated with the stimulating light beam reflected by the dichroic mirror. In addition, sharpness of the image information may also be improved as the image information reading apparatus of the optical head scanning type detects only the fluorescence induced at a spot currently irradiated with the stimulating light beam and does not detect afterglow of the fluorescence from adjacent areas where the irradiation has already been completed.
However, the image information reading apparatus of the optical head scanning type also has a problem. The problem occurs because the scanning target which carries the image information to be read may be any of a gel-like supporting object without glass, a membrane filter onto which the gel-like supporting object is transcribed, or an accumulation phosphor sheet, instead of the gel-like supporting object with glass described above. Though the fluorescence would be emitted from the upper surface of the scanning target as shown in FIG. 5 if the scanning target was the gel-like supporting object with glass, the fluorescence might be emitted from other parts in the scanning target if the scanning target was one of the other types listed above.
For example, the fluorescence would be emitted from any part along a thickness dimension of the scanning target if the scanning target was the gel-like supporting object without glass, or would be emitted from a lower surface of the scanning target if the scanning target was the accumulation phosphor sheet. Concerning the membrane filter, the fluorescence is emitted from a lower surface thereof the same as the accumulation phosphor sheet. Moreover, though a major portion of the fluorescence emitted from the accumulation phosphor sheet etc. originates from the lower surface thereof, the surface which corresponds to a plane of incidence for the stimulating light beam, a small fraction of the fluorescence may be emitted from planes above the lower surface.
Thus, as illustrated in FIG. 6, a distance f between a light-emitting plane of the scanning target 10 and an objective lens 53 provided in the optical head 50 may vary when the type or thickness t of the scanning target 10 is changed. For this variation of f, the objective lens 53 fails to sufficiently collimate emitted light K, e.g. the fluorescence, emitted from the light-emitting plane of the scanning target 10. Finally, the emitted light K reaches the photomultiplier 40 as diverging light (especially when the distance f between the objective lens and the light-emitting plane is shorter than a focal length f0 of the objective lens).
As the amount of light incident on the photomultiplier 40 varies according to distance between the optical head 50 and the photomultiplier 40, the amount of light detected by the photomultiplier 40 may change when a scanning position of the optical head 50 is shifted even if the original light amount emitted by the scanning target remains constant, which may cause a problem of non-uniform density in the image information S.
Besides, uniformity of the density of the image information S would be further deteriorated if the scanning target 10 was one which emits the fluorescence at any part along the thickness dimension thereof, e.g. the gel-like supporting object without glass, as thickness of a certain portion of the scanning target, the portion from which detectable fluorescence is emitted, would change as the scanning position of the optical head is shifted.
The object of the present invention is to provide an image information reading apparatus of the optical head scanning type, which is designed to maintain the amount of detected light originating from a scanning target of any thickness and any type at an uniform level when the scanning position of the optical head is shifted.
To attain this object, the image information reading apparatus of the present invention is provided with an optical element which is capable of collecting light, e.g. a collective lens, between an optical head and photoelectric reading means thereof in order to compensate for a gap between a focal length of an objective lens of the optical head and distance between a reading plane of the scanning target and the objective lens, this gap perhaps being caused because of the scanning target having a thickness different from a predetermined thickness.
The image information reading apparatus of the present invention is constituted of a light source for emitting a light beam, the photoelectric reading means for detecting intensity of incident light in an photoelectric manner, a sample holding portion for holding the scanning target carrying certain image information, the optical head which leads the light beam emitted by the light source to the scanning target placed on the sample holding portion so that the scanning target is irradiated with the light beam and which leads induced light emitted by the scanning target in response to the irradiation thereon to the photoelectric reading means, primary scanning means for moving the optical head in one direction (a primary scanning direction) with respect to the scanning target so that the scanning target is scanned in the primary scanning direction with the light beam led thereto by the optical head, secondary scanning means for moving at least one of the optical head or the sample holding portion in another direction (a secondary scanning direction) which is substantially orthogonal to the primary scanning direction, and the optical element which is provided at a part on an optical path of the induced light between the optical head and the photoelectric reading means and which has a sufficient refractive power for collecting the induced light led toward the photoelectric reading means by the optical head.
The sample holding portion includes a sample table etc. for holding the scanning target as a main constituent thereof. Whatever constitution the sample holding portion may have, any of the gel-like supporting object, the membrane filter onto which the gel-like supporting object is transcribed, the gel-like supporting object with glass, a slide glass, a microtiter plate, the accumulation phosphor sheet or another similar sample used in the fields of biochemistry and molecular biology can be adopted as the scanning target. Accordingly, one example of the scanning target carrying the certain image information may be the gel-like supporting object in which the organic substance marked with the fluorescent dye is distributed.
The term xe2x80x9corganic substancexe2x80x9d as used herein refers to any substance related to organisms and covers any substance which may act as a hormone, a tumor marker, an enzyme, protein, nuclear acid, antibody or antigen, and cDNA and mRNA of any kind.
The term xe2x80x9cinduced lightxe2x80x9d as used herein refers to any kind of light emitted by the scanning target in response to the irradiation and covers simple reflected light, the fluorescence induced therein, light induced by any type of stimulation, etc. The light beam with which the scanning target is irradiated may be a beam of simple illumination light, or may be a beam of stimulating light, e.g. a laser beam, which is capable of stimulating a simple phosphor or a phosphor which emits light under certain stimulation.
It is desirable to have the secondary scanning means capable of moving the optical head, and have the optical element constituted of at least a first optical element to be moved together with the optical head by the secondary scanning means and a fixed second optical element. This is because the level of the light amount to be detected can be made even more uniform over all of the scanning positions of the optical head when a plurality of optical elements are used.
The optical element used in the present invention may be a lens with a positive refractive power, a parabolic mirror, a concave mirror, etc. The photoelectric reading means in the present invention is preferably, but not limited to, a photomultiplier or a similar device which is capable of detecting the induced light emitted from the scanning target with high sensitivity, wherein it is also possible to adopt well-known photoelectric reading means of another kind such as a cooled CCD.
According to the image information reading device of the present invention, the light amount of the induced light incident on the photoelectric reading means may be maintained at a substantially uniform level regardless of the scanning position of the optical head, by providing the optical element between the optical head and the photoelectric reading means in order to compensate for the gap between the focal length of the objective lens in the optical head and the distance between the light-emitting plane of the scanning target and the objective lens, the gap which may be produced when the thickness or the type of the scanning target is different from the predetermined one.